The objectives are: 1) to estimate the probabilities of detecting a breast cancer by mammography and physical exam and to find how these probabilities change with cancer size; 2) to estimate the true age-prevalence curve for breast cancer in screened populations and the number of breast cancers of specified sizes that are missed by screening in one year; 3) to identify characteristics of women that make them especially easy or difficult subjects in which to screen breast cancers; 4) to obtain information about the frequency of breast cancers that are especially difficult to ascertain by screening. Data will be from five selected BCDDP screening programs. The underlying parameters, the probabilities of detecting a breast cancer by mammography and physical exam, determine the pattern of mode of detection by screening (mam. only positive, phys. only positive, or both positive) and can be estimated by maximum likelihood. The relationship of cancer size to probability of detection will be determined. Since the proportion of missed cancers is 1- (proportion ascertained by screening), the prevalence of breast cancer in the first year of screening is obtainable from the estimated probabilities. Additional factors that will be examined include Wolfe's pattern, menopausal status, cup size, weight, age, race, micro-calcifications, and the difference between the first and later screening years. A preliminary study has validated the assumptions of the statistical analysis and given tentative estimates of the basic parameters. Data from five Breast Cancer Detection Demonstration Projects have the necessary data computerized and are available for this analysis.